THE GENETICS OF MATING TYPE IN A
SYNGEN OF GLAUCOMA*
PAULA LEE CHO
Zoology Department, University of Illinois, Urbana 61801
Received October 12, 1970
REEDING in Ciliates takes the form of conjugation. In the mating reaction,
animals unite in pairs, undergo meiosis, fertilize each other, and separate.
Then both exconjugants of each pair multiply by repeated fissions. Conjugation
occurs when cells of complementary physiological classes (mating types) are
1954). Within a particular taxonomic species of Ciliate,
mixed ( SONNEBORN
there may be several sets of complementary mating types (syngens) which cannot interbreed among themselves. Cells within each syngen constitute a po1957). The distinction between cells
tentially common gene pool ( SONNEBORN
of different mating types has a physiological basis and may not be morphologically identifiable.
The purpose of this paper is to report on the genetics of mating-type determination in strains of the Glaucoma chattoni-scintillans group. Some of these
strains conform to CORLISS’
(1953a) description of either G. chattoni or G. scintillans, but some are intermediate (CHO 1971). In a previous study on these forms,
PHILLIPS
and ABRAHAM
(1969) reported two mating types in syngen 1, and
observed that a pair of exconjugant clones (a synclone) usually has identical
mating type: mating types I and I1 appeared in an approximately 1 : 1 ratio in
successive generations. They inferred that this pattern reflected a simple genic
mating-type inheritance in which the mating-type phenotype is determined
directly by the genotype. Fragmentary results on crosses in syngen 2 suggested
that expression of the mating-type phenotype might involve epigenetic factors
which modify or limit genotypic pluripotency.
In Japan, NAKATA
also observed mating types in Glaucoma scintillans (NAKATA 1969). He confined his crosses to mating types V and VI1 of his syngen 1.
(There is no correspondence between the numbering of the Japanese syngens
and the American syngens; and no attempt could be made to determine the relationship since NAKATA’S
lines were lost prior to publication.) He also observed
that exconjugants of the same pair were alike in mating type. i.e., showed synclonal uniformity. However, the mating types of the F, generation were VI and
VI1 in a ratio of approximately 1:1. The mating type V phenotype was not recovered in the F, progeny.
The present study attempts to extend our understanding of the mode of
This report is part of the research conducted in partial fulfillment of the requirements for the degree of Doctor of
Philosophy at the University of Illinois, Urbana. The work is supported hy Grant GhI-07779 from the U. S. Public
Health Sellice to D. L. NANNEY.
Genetics 6 7 : 377-390 March, 1971.
3 78
P. L. CHO
mating-type inheritance in the American syngen 1. The results are similar to
and ABRAHAM
and of NAKATA
in showing that mating type is
those of PHILLIPS
inherited in a synclonal pattern. Further work shows that the inheritance of
multiple mating types is under direct genic control at one locus with a series of
alleles arranged in order of dominance.
MATERIALS A N D METHODS
Five syngens were found among the collections. Their mating relationshps are shown in
Table 1. Strains 4 and 8 (mating types I and 11) of syngen 1 were collected from Urbana, Illinois.
Strains of syngen 2 were collected from Ottawa, Illinois. These were described by PHILLIPS
and ABRAHAM
(1969). Strains of syngen 3 were collected from Brown County, Indiana and identified by Dr. DAVIDNANNEY.
A strain of mating type I1 (ST6) of syngen 1 was isolated from
another collection in the vicinity of Urbana, Illinois by the author. The author also collected
strain CL (mating type I1 of syngen 1) from Crystal Lake in Urbana. In another collection at
the same location, the strains of the three other mating types of syngen 1 (111, IV, and V) were
identified. Strains of syngens 4 and 5 were collected at yet another date by the author at the same
site.
To set up cultures, a few grains of rice were added to each 57 g bottle collected to enrich
the growth of Ciliates. One day later, single cells were isolated from each bottle and grown in
0.15% Cerophyl medium (pH adjusted to 7.0 with NaOH) inoculated with Aerobacter aerogenes
the day before. Clones which grew well were placed in test tubes with 2 ml of the medium two
days later. Stocks were kept in these small test tubes at room temperature. Weekly transfers
were made to maintain them. A parallel series of stocks was preserved at 15°C and was fed once
a month. Periodic checks on viability and mating ability when these cold-room stocks were returned to room temperature revealed no apparent difference.
To initiate mating, one drop of cells of each of two mating types growing exponentially was
added to a depression slide. 0.3 ml of the medium diluted 1:l with demineralized water was
added. Pairs were observed 16-20 h r later.
A micropipette was used to pick out pairs of conjugating cells. Each pair was placed i n the
center depression of a 3-spot depression slide. 0.15 ml of the culture medium diluted with 3 parts
of water was added. 18-24 hr later, when all the conjugating pairs had separated, one exconjugant was delivered to each side depression on the slide and 0.3 ml of the 50% medium was added.
The depressions were checked for growth every 3 or 4 hr. When cell divisions were observed,
undiluted medium was added to fill the depressions.
Cells which had undergone true conjugation were usually unable to mate again for several
cell generations. In order to hasten the onset of maturity, a series of single-cell transfers of each
line was made every other day and the line was tested for maturity at each transfer.
The test for maturity involved mixing a log phase culture of the line concerned with each
of the five testers in separate depressions. A control was maintained to which no tester was added.
0.3 ml of the 50% medium was added to each depression. The slides were examined for pairs
after 18 hr and again every 3 hr later up to 27 h r to allow for detection of mating despite differences in time of initiation of mating. Since conjugating pairs stayed together for about 8 hr,
this periodic examination should uncover any pairs formed and serve as a check on false positive
observation. If pairs were observed in the control, the line was classified as a selfer and the test
was repeated on further transfers. The line was classified as a particular mating type (X) if it
mated with all four other mating types but not with X.
A sample of each line isolated from nature was stained with a silver-impregnation technique
of Chatton-Lwoff (CORLISS
1953b) to identify the genus. In order to study the nuclear events
taking place during conjugation, samples of cells were taken from a mating culture at I-hr intervals and stained with Schiff’s reagent using the technique developed by DIPPELL (SONNEBORN 1950). The cells were counterstained with fast green to give better contrast.
379
M A T I N G T Y P E IN GLAUCOMA
RESULTS
Zdentification of the five syngens: Some Glaucoma cells were isolated from
every collection. However, not all the strains mated. This is not surprising since
mating occurs only when mature cells of complementary mating types from the
same syngen are mixed. If only one type of a certain syngen is collected, no
mating would be expected to occur between it and any other strain.
At least three additional syngens have been identified in this study (see Table
1 for details). Syngens 1 and 2 had been described by PHILLIPS
and ABRAHAM
(1969). In addition to the two mating types (I and 11) of syngen 1 they deTABLE 3
Mating reactions of the five syngens of Glaucoma
Syngcn
3
2
1
Nun]b e r
4
5
Mating
Type
I
I1
Strain
4
8
I11 IV V
CLB
CLF
ST6 CLH
CLI
CLN
CLO
CLP
CL
I
I1
111 I
I1
111
I
-
5 pairsldepression
10 or more pairs/depression
I1
CL3 CL8 CL72
CL5 CL15 CL19
CL6 CL18 CL21
CL7
CL9
CLlO
CLll
CL17
CL22
CL23
CL25
-
I
+
I1
CLA CZC S F 1 SF2 SF8 M 1 1 1322 M23 CLI CL4
CLD CLJ
M12
CL7 CL24
CLE CLK
M13
C1.14
M14
CL: 5
CLL
CLM
M4 1
CL23
M4 2
14.43
syn5
I1
+I- 1
I11 I
+
380
P.
L.
CHO
scribed. three other mating types were isolated: 111, IV, and V. The three mating
types previously described for syngen 2 were not reencountered. Three mating
types are here first reported in syngen 3. Three mating types were also found in
syngen 4, but only two in syngen 5. Further collections might well add new
mating types to most of these syngens and new syngens as well.
Some intersyngenic mating reactions were observed. In all cases, however,
intersyngenic mating reactions were much weaker than intrasyngenic reactions.
The number of cells which actually formed pairs was small. When these pairs
were isolated, they were found to be nonviable. but as will be seen, intrasyngenic
crosses also yield many nonviable pairs.
Siluer impregnation technique: The silver-stained slides of the cells confirmed
that they were Glaucoma by CORLISS’
criteria (CORLISS1953a). The buccal
overture was tilted to the right at an angle to the meridians. The oral membranelles consisted of an undulating membrane with three adoral zone membranes and were arranged as described by KLUC (1968). The number of meridians varied from 27 to 37. A detailed cortical analysis of some of the strains is
being presented elsewhere (CHO1971).
Feulgen technique: A timed study was made to observe the nuclear events
during and after conjugation. The nuclear events are very similar to those in
Tetrahymena pyriformis described by NANNEY
(1953a). The vegetative cell has
one macronucleus and, in close proximity, one micronucleus. When conjugation
occurs, the two cells attach at the anterior ventral surface close to the oral apparatus. The micronucleus moves anterior to the macronucleus, enlarges, and
elongates to form the “crescent” stage. ’The nuclei undergo their first prezygotic
division during which the chromosomes appear as fine lines inside a clear area.
At metaphase of this division, the number of chromosomes (“tetrads”) appears
to be less than ten. This stage is followed by the second prezygotic division which
results in four nuclei.
Three of the four prezygotic nuclei of each cell move posteriorly and eventually disintegrate. The fourth one attaches to the membrane between the cells
and undergoes another division. One of the daughter nuclei remains at the membrane (the migratory nucleus) and then migrates to the other cell, whereas the
other (the stationary nucleus) passes posteriorly and remains close to the macronucleus. Fertilization is accomplished by fusion of the stationary nucleus with
the exchanged migratory nucleus. The two exconjugants should have the same
nuclear genetic constitution. The zygote nucleus undergoes two postzygotic divisions. The two nuclei at the posterior end retain their staining property and
develop into micronuclei. Further movements result in the two micronuclei being situated between the two macronuclear anlagen. The old macronucleus becomes irregular in shape and begins to degenerate. Eventually the two exconjugants separate. Each exconjugant thus contains two macronuclear anlagen
and two micronuclei.
One of the micronuclei disintegrates while the other undergoes a third postzygotic division, resulting in two micronuclei which, with the two macronuclei,
are distributed equally at the next cell division. Each of these two cells initiates
a caryonide (asexual progeny with macronuclei of common descent).
381
M A T I N G TYPE I N GLAUCOMA
TABLE 2
Viability of exconjugants
B?th
Observed
Expected
exconjugants
One
erconjugant
live
lives
1
0.4
8
Both
exconluganis
die
51
50.5
9.1
x2
Nonconiugants
.
-
10
= 1.03, 0.3 < P
Total
exconjugants
. .
60
60.0
< 0.5
Results of the crosses: The mode of mating-type inheritance was found to be
synclonal; that is. both exconjugant clones exhibited the same mating type at
maturity. This synclonal inheritance was consistent both with the observations
and ABRAHAM
(1969) on syngen 1 and also with those of NAKATA
of PHILLIPS
(1969). The viability in these mating experiments was very low. Depending on
the individual cross, it varied from 0-15 %. Most death occurred without division
taking place after the exconjugants separated. Table 2 shows that death occurred
randomly among the exconjugants. That is, there is no excessive number of pairs
in which both exconjugants die, nor is death predominantly unilateral.
After conjugation, some exconjugant clones went through an immaturity period during which they were unable to perform conjugation when mixed with
and ABRAHAM
(1969) found no
competent cells of other mating types. PHILLIPS
immaturity period in crossing strains of mating types I and I1 of syngen 1. In
the present study of crosses among three other mating types, a variable immaturity period was found (Figure 1 ) . A small proportion (2%) of the cells
'
w
:
90-
80-
P,
3
70-
$
60-
0
50-
\
r,
c
40-
U
30 -
P,
<
<
20-
IO 20 30 40 50 60 70 80 90 100 110 120
Number of Cel I Divisions
FIGURE
1.-Percentage
fissions ( N = 96).
of cells reaching maturity at each transfer. One transfer equals ten
3 82
P. L. CHO
TABLE 3
Results of crosses between CLP (mating type I l l ) and CLC (mating t y p e V )
Mating
Type 11
1. F,
58
+ 4*
2. F,
a. CP156a x CP161
10 0'
b. C P 1 2 x CP13
2flf
3. Backcrosses of parent with F,
a. CLC (mating type V) x CP161
(mating type 11)
5 0*
b. CLC x CP156a (mating type 111)
0
+
+
Mating
Mating
Type 111
Type 1V
72f I *
0
7 (2)t
+ 1'
+ 4'
2
1
6 (1)t
11
8
0
6 f0'
0
4 + I'
5'
10
+
Selfers
1
5
* Addition due to selfers which stabilize to become this mating type.
-f The number in parentheses represents selfers which died without stabilization.
were mature as soon as they could be tested after conjugation. Half of the cells
became mature between 10 and 30 fissions. Another 40% became mature during
the next 70 fissions. 10% were still immature after 100 fissions.
Using strains CLP (mating type IIJ), CLL (mating type IV), and CLC (mating type V) as the parents, three sets of crosses were set up:
1. CLP (mating type 111) x CLC (mating type V) , see Table 3.
a. I n the F, generation, no mating type V clone was recovered. Combining the
data in which both exconjugants were viable with those in which only one was
viable. 58 pairs were mating type I1 and 72 pairs were mating type 111. Two of
the seven selfing clones died without stabilization, four stabilized to become
mating type 11, and one stabilized to become mating type 111.
b. I n the F, generation, two different crosses were made. In one cross (b) , no
mating type V clone was recovered. Type I11 was produced in excess. A similar
cross (a) using two different F, clones yielded three mating types: 11,111,and V.
c. In a backcross to CLC (the mating type V parent) a type I1 F, gave types
I1 and V in about equal frequencies. A type I11 F, in a similar backcross produced types I11 and V.
2. CLP (mating type I11 x CLL (mating type IV) ,see Table 4.
a. In the F, generation, only types I1 and IV appeared. Of the 15 selfers, three
Stabilized to become mating type 11, and 12 stabilized to become mating type IV.
b. In the F, generation, pairs isolated from selfers of the F, generation contained types I1 and IV. Crosses between F, pairs yielded similar results.
c. Backcrosses were made to CLP (the mating type 111parent). The backcross
with a type I1 F, clone yielded all mating type 11clones. One backcross to a type
IV F, clone yielded mating type IV clones and a selfer which stabilized to become mating type 11. Another similar cross with a different type IV F, clone
yielded both types I1 and IV clones.
3. CLL (mating type IV) x CLC (mating type V), see Table 5.
a. In the F, generation, only types I1 and IV were recovered. Of the 6 selfers,
one died without stabilization and the other five stabilized to become mating
type IV.
383
M A T I N G T Y P E IN GLAUCOMA
TABLE 4
Results of crosses between CLP (mating type Ill) and CLL (mating type IV)
Mating
Mating
Type 11
1. F,
28
Type IV
+ 3*
34
Selfers
+ 12*
15
2. F,
a. Selfers
b. Cross between F, pairs
3. Backcrosses of parent with F,
a. CLP (mating type 111) x LP9e
(mating type 11)
b. i) CLP (mating type 111) x LP9a
(mating type IV)
ii) CLP x LP5b (mating type IV)
4+O*
9+l*
2 + 1'
6 + 1'
2
1
8
0
0
O+l*
1
4+O*
4
1
0
* Addition due to selfers which stabilize to become this mating type.
b. From the F, selfer (CL47s), type IV clones and selfers stabilizing to become mating type IV were obtained.
Different crosses between different F, clones were attempted (mating type
I1 x mating type IV) . However, only one viable progeny (type IV) was obtained out of 1000 pairs isolated.
Selfers: Two kinds of selfers may be distinguished. The first kind was found
among cells which divided early (within 24 hr) after conjugation. Most of the
selfers were in this category. Table 6 shows the results of one experiment in
which 13 of the 14 selfers found were among the early dividers. Most of these
TABLE 5
Results of crosses between CLL (mating type IV) and CLC (mating type V )
Mating
1. F,
Mating
Type I1
Type IV
Selfers
14 f O*
20'+ 5'
6 (I)+
2. F, (selfers CL47s)
0
1 +9*
9
* Addition due to selfers which stabilize to become this mating type.
+ One selfer died without stabilization.
TABLE 6
Number of selfers among cells which divided early and those which diuided late
Early dividers
Late dividers
Total
Selfers
Nonselfers
Total
13
39
52
1
57
58
96
110
14
x2 = 13.38, one df, P
> 99.95
384
P. L. C H O
selfers stabilized within 10 cell divisions; a few stayed immature for 10-20 fissions. Their mating type. when stabilized. remained the same as the parental
line from which they derived, i.e., their mating types were parental rather than
synclonal. (These selfers were regarded as nonconjugants in reporting the above
Results of crosyes.)
In order to study the characteristics of these early selfers, 12 lines were isolated from each strain. 2-4 out of the 12 lines in each series died. The rest of the
12 lines all had the same mating type at stabilization.
The second kind of selfer was found among the late dividers. (These were the
selfers reported under Results of crosses.) After stabilization. their mating-type
characteristics resembled those of their coconjugants which were usually nonselfer;. The two exconjugants thus ended up with the same mating type (synclonal inheritance patterns). When 12 lines were isolated from each strain of
selfers, 2-4 out of 12 died while again all the rest had the same mating type at
stabilization.
DISCUSSION
Znheritance of maling type: The inheritance of mating type in Ciliates often
involves a complex system in which the genotype circumscribes the range of
mating-type potentiality. Cellular differentiation then confines a cell to one
mating type. This pattern of expression, once established, may be maintained
as a “hereditary” trait throughout fission; under special circumstances, unexpressed potentialities may, however, be brought to expression.
The most prominent example for this kind of mating-type inheritance is
found in the group A caryonidal system in syngens 1. 3, 5, 9, and 11 of Para1937, 1939) as well as Tetramecium aurelia (KIMBALL1937; SONNERORN
hymena pyriformis, syngens 1. 3, and 7 (ALLEN1956; BYRD1959; NANNEY
1956,1959; NANNEY
and CAUGHEY
1953,1955; NANNEY,
CAUGHEY
and TEFANKJIAN 1955; PHILLIPS
1969). After conjugation or autogamy, two macronuclear
anlagen are formed from each syncaryon and are distributed to the two daughter
cells (caryonides) . At maturity, caryonides have ordinarily been determined to
exhibit one of the mating types limited by the genotype. There is no more tendency for the two caryonides to have the same mating type than expected on a
chance basis. The frequencies of the different mating types are dependent on the
genotype and are affected by temperature.
In the group B syngens of Paramecium aurelia, i.e., syngens 2, 4, 6. 7, 8, 10,
and 12 (NANNEY
1954, 1956; SONNEBORN
1954; TAUB
1959), caryonidal inheritance of mating type is obscured by the major role of the cytoplasm. Excon
jugant clones generally retain the same mating type following conjugation
Changes of type ordinarily occur only when cytoplasmic exchange takes place.
In the simpler systems of genic inheritance, there is a one-to-one correspondence
between the mating-type geno ty-pe and phenotype. Generally, the genotype determines the mating-type phenotype directly without the intervention of epigenetic effects. Genic inheritance involving loci with codominance was found in
E u p l o f ~ patellu
s
(KIMBALL1942). A system of dual alleles with dominance at
M A T I N G T Y P E IN G L A U C O M A
385
multiple loci was found in Pnramecium bursaria (BOMFORD
1965; SIEGEL1963).
Dual alleles with dominance at a single locus was found in Paramecium cau1964). Mating-type inheritance under direct genic
datum, syngen 3 ( HIWATASHI
control involving multiple alleles which may be serially arranged in order of
dominance has been established or indicated in Tetrnhymena pyriformis, syngens
2 and 8 ( ORIAS1963) Euplotes unnnus (HECKMANN
1961, 1963), Euplotes crassus (HECKMANN
1964), and Euplotes minuta (NOBILI1966). The following
analysis demonstrates that Glaucoma also shows genic inheritance of mating
type.
The crosses have some puzzling features. and analysis is hampered by the low
viability, but a system of mating-type inheritance may be deduced. The synclonal
pattern of mating-type inheritance indicates that the mating types are controlled
directly by the genes. A one-locus scheme with dominance can account for the
observations. More specifically. we may postulate that each mating type is represented by a different allele and that the mating type TV allele (mtrv)is the most
dominant, with mating type I1 (mt"). mating type IT1 (mtll') and mating type
V (mt") following in order of decreasing dominance. When two nonidentical
alleles are present in the same organism, only the more dominant is expressed.
The position of mating type I in the hierarchy is not discernible by this study.
This interpretation is qualitatively consistent with all the data, with one striking exception. Strain CLP behaves in crosses as if it were mt"/mt". The rules
place mt" higher in the dominance sequence than mtnr,yet CLP is phenotypically
type 111.We will return to this anomaly later.
I n the first cross, involving CLP (mating type 111) x CLC (mating type V).
the postulated parental genotypes are mtl'/mtr" and nit'/mt", respectively. TWO
types of progeny are produced in the F, generation: mt"//mtv and mtl"/mtT'.The
former expresses mating type I1 and the latter mating type 111. No mating type V
cells should be found in the F, generation. The results show 58 mating type I1
clones, 72 mating type I11 clones, and seven selfers, but no mating type V cells.
Crosses between F, cells would represent (mt"/mtv x mt"'/mt"). The F, cells
should be mt"/mtv. mtlr/mt'll,mtIr1/rntv,and mtv/mtv.The phenotypes expected
are mating type 11. mating type 111, and mating typc V in the ratio 2: 1: 1. All
three mating types were recovered in one of the crosses between F,'s. even though
type V did not occur among 16 pairs in the other cross. In backcrosses of the type
I1 F, to the type V parent, the progeny should be mP/mtv and mtv/mt'; five
mating type I1 pairs and four mating type V pairs were obtained. In the similar
backcross of the type I11 F,, the expected mating types are mtl"/mtv and mtv/mt'.
Observed are six mating type I11 pairs to 10 mating type V pairs.
The second series of crosses in Table 4 is also consistent with the hypotheses.
Starting with the CLP (mtrl/mt"') and CLL ( mtl'/mtl") parents, the F, generation should consist of mt"/mtrT,mtrr/mtnr,
mtTr/mt1",
and mt"'/mt". The first two
classes would yield mating type I1 cells, whereas the last two classes would yield
mating type IV cells. 28 mating type I1 pairs and 34 mating type IV pairs were
obtained in the F, generation.
There are four different possible combinations in the F, generation. In the first
386
P. L. C H O
one (mt"/mt" x mtf1/mtIT')
, the progeny would be mtN/mt" and mt"/mtrv and
phenotypically mating type I1 and mating type IV. Phenotypically identical
results would be produced in the crosses of (mt"/mt" x mtl"/mtIv) and (mtrJ/
mt'" x mt"/mtrV).Only in the combination of (mtIr/mtl" x mP"/mt1') would
% of the progeny be mating type 111 in addition to mating type IV and %
mating type 11. Thus, in this limited combination of F, crosses, only 13 mating
type I1 and eight mating type IV pairs were recovered.
In the backcrosses with the m P / m P parent, there are two possible combinations with the mating type I1 cells. The first is (mtrr/mt" x mtrr/mt"') yielding all mating type I1 progeny. This was indeed observed in the results. The other
combination, the anomalous parent mtl'/mt"' (I11 phenotype) x F, mtrl/mtrrr
(I1 phenotype). would yield % mating type I11 progeny.
In the other backcross of F, mating type IV cells with the mtrl/mtrl'parent, the
first combination (mtr'/mt'" x mt"/mtlv) would yield only mating type I1 and
mating type IV progeny. This was indeed observed. The other combination
(mtI'/mt"' x mt'"/mt'') would yield mating type I11 progeny.
The third series of crosses was made with CLL ( mP1/mtrv)x CLC (mtv/mtv).
Two mating types are expected in the F, cells, namely mtrr/mtv(mating type 11)
and mt''/mt' (mating type IV). The results showed 14 mating type I1 cells to
20 mating type IV cells. It was not possible to obtain enough viable cells from the
F, crosses and backcrosses to conduct a meaningful analysis of the F, generation.
It is interesting to note that crosses made on syngen 1 by PHILLIPS
and ABRAHAM (1969) also fit this scheme, although the position of the mt' gene in the
dominance scale is not known.
Assuming that mtl is recessive to mtrl,the parental genotypes are (mtr/mtlx
mtl/mtr'),the F, and F, cells would both contain mtl/mt' and mt'/mt'' cells in
the ratio of 1: 1. This was indeed observed. On the other hand, if mtl is dominant
to mt", then with the parental genotypes of (mt'/mt" X mtr'/mP'), the F, and F,
cells would be mtl/mtl' (mating type I) and mt"/mt" (mating type 11) which
would also be consistent with their results.
The anomalous mating type of strain CLP: The breeding analysis of strain CLP
establishes it as a heterozygote, nd1/mt1Ir,
and all the data are consistent in indicating that the mt" allele is dominant. Yet CLP manifests type 111. Two kinds of
explanation are a priori possible. Perhaps a 5-eversal of dominance" has occurred,
either because of pecularities of the strain's genetic constitution or circumstances
of the cell's environment at some point in its history. Alternatively, the anomaly
might not be resolved in the realm of genetic physiology but in a cytogenetic
aberration. Breeding results bear on the micronuclear genotype. Although micronuclei and macronuclei are ordinarily concordant genetically, SONNEBORN
(1947) was able long ago to artificially construct cells which were micro-macro
heterocaryons in Paramecium aurelia. More recently, COLEand SIEGEL(1969)
explained some peculiarities in the genetics of Paramecium bursaria on the basis
of such naturally occurring heterocaryons. ALLEN(1967) has also found systematic maintenance of parental macronuclei with recombined micronuclei in some
crosses in Tetrahymena pyriformis. Perhaps CLP has a macronucleus lacking the
MATING TYPE I N GLAUCOMA
387
higher allele, and only a micronucleus with the mt“ allele. This situation would
account for the observations.
Selfers: The “early” selfers in Glaucoma exhibited selfing shortly after conjugation. It lasted for a few fissions after which the parental mating type was again
established. This behavior can most easily be interpreted in terms of a phenomic
lag combined with “nonconjugation.” In this scheme, early selfing is due to an
exchange of cytoplasm between cells of different mating types during an abortive
conjugation. As fissions proceed, the parental macronucleus resumes activity and
the exchanged cytoplasm is diluted out to progeny cells; the expression of mating
type regains stability and selfing disappears. Massive cytoplasmic exchange at
conjugation occurs regularly in Tetrahymena pyriformis (MCDONALD
1966) and
can be induced in Paramecium aurelia (SONNEBORN
1947). Its occurrence in
Glaucoma has not been examined.
The “late” selfers in Glaucoma could be explained in more than one way. Since
only the dominant mt allele is expressed at maturity, the dominant allele might
actually repress the action of the recessive allele. The dominance pattern could
reflect a polarized allelic repression. If so, selfing might be a transient developmental event. If, during maturation the recessive allele is occasionally turned on
prematurely, before the dominant allele is activated, the recessive allele could be
expressed for a brief period. After activation of the dominant allele, a transition
period would be expected during which the clone would contain cells of both
mating types and/or ambivalent cells. Later, all cells of the clone would express
the dominant allele.
The “late” Glaucoma selfers are clearly different from several other kinds of
selfers known in Ciliates. They are transient selfers and are therefore distinct
from the “persistent” selfers of the AA strains of Tetrahymena pyriformis
(ELLIOTT
and NANNEY
1952) and many other Ciliates. Their pattern of stabilization sets them apart from the “assorting” selfers studied so extensively in syngen
1 of Tetrahymena pyriformis (ALLEN1956; ALLENand NANNEY
1958; NANNEY
and ALLEN1959; NANNEYand CAUGHEY
1953, 1955). The Glaucoma selfers of
a particular clone all stabilize to the same genetically rationalized type. To explain them, one does not have to resort to genetic or epigenetic hereditary factors
in either the macronucleus or the cytoplasm (cf. SIEGEL1970) . The closest parallel is perhaps certain kinds of selfers in syngens 5 and 7 of Paramecium aurelia
(BLEYMAN1967; TAUB
1966), and a developmental interpretation in some form
seems most appropriate.
The author wishes to express her sincere thanks to Dr. DAVIDL. NANNEY
for stimulating discussion of this work and for valuable criticisms during the preparation of the manuscript.
SUMMARY
Five syngens were found among strains of Glaucoma (Ciliata) collected in
Illinois and Indiana. Study of syngen 1 revealed a synclonal pattern of matingtype inheritance. The mating-type trait appears to be controlled by one gene
( m t )with five alleles serially ordered for dominance.
388
P. L. CHO
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